If the volume of the tank is, say, V litres, then
A fills it at a rate of V/3 litres/hour
B fills it at a rate of V/4 litres/hour
C fills it at a rate of V/6 litres/hour
So A+B+C would fill it at a rate of V/3 + V/4 + V/6 = 3V/4 litres/hour
Time taken = V/(3V/4) = 4/3 hours or 1hour 20 minutes.